His resurrection ability is irreplaceable and exceedingly powerful. With this ability alone, he is the most formidable support personnel. So, how could he possibly be sent to the frontline? That's the first reason.
Secondly, Chen Mo is now a treasure to all of Earth's civilization and the only individual in the entire Milky Way Sector possessing a Chaos Supreme Treasure. All aliens harbor a deep-seated desire to eliminate him first and foremost. If he were to appear on the front line, opponents might even abandon their scattered attacks on the Divine Domains and instead concentrate all their combat power to encircle and eliminate him. For this reason alone, Chen Mo absolutely cannot be allowed on the frontline.
